Astron. Astrophys. 348, L45-L48 (1999)

1. Introduction

The early type O and B star winds are well described by line-driven wind theory (Lucy & Solomon 1970; Castor et al. 1975; Pauldrach et al. 1986; Friend & Abbott 1986), and this theory also explains the observed X-ray emission from hot stars as arising from wind shocks (Lucy 1982; Cassinelli & Swank 1983; Owocki et al. 1988). However the evolved massive Wolf-Rayet (WR) stars are different from O and B stars, in that the momentum of WR winds typically exceeds the single scattering limit by an order of magnitude (e.g., Barlow et al. 1981). At present, the most promising model for accelerating the high mass loss WR winds derives from multiple scattering by lines (Lucy & Abbott 1993; Springmann 1994; Gayley et al. 1995). Importantly, Gayley & Owocki (1995) have shown that even with multiple scattering, the instability mechanism that leads to shock formation in the lower mass loss OB star winds should still operate in the WR winds, and so potentially account for the observed X-ray emission.

The first quantitative X-ray information on WR stars was obtained with EINSTEIN (0.2-4.0 keV) by Seward & Chlebowski (1982). Pollock (1987) has summarized the EINSTEIN results for 48 WR stars, which consist mostly of low S/N broadband measurements. He noted that N-rich WR stars (WN) tend are more luminous than the C-rich WR stars (WC) on average, suggesting as a potential explanation the different chemical compositions between the two types. The ROSAT All-Sky Survey (RASS) has provided PSPC broadband fluxes for nearly all galactic WR stars (Pollock et al. 1995). The softer ROSAT response (0.2-2.4 keV) has led to only few quantitative results on the generation and production of shocks in WR winds, with spectra existing only for WR binaries. However, the ROSAT data has revealed that unlike the O stars (e.g., Kudritzki et al. 1996), the X-ray luminosities of single WN stars are not correlated with Bolometric luminosity , wind momentum , wind kinetic luminosity , or WN subtype (Wessolowski 1996). Although fewer in number, likewise there are no such correlations for 17 single WC stars (see Fig. 1). This paper concerns the interpretation of these observations from considerations of scaling relations.

Fig. 1. A plot of ROSAT X-ray luminosities for WN (squares) and WC (triangles) stars vs. Bolometric on left and density scale on right. No systematic trends are apparent, except that on average. (Values of , , and from Koesterke & Hamann 1995 and Hamann & Koesterke 1998.)

© European Southern Observatory (ESO) 1999

Online publication: July 26, 1999
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